Project 2 focuses on the ability of BDNF (brain-derived neurotrophic factor) in the NAc (nucleus accumbens) and VTA (ventral tegmental area) to regulate mood and motivational state. Most work in the field (including considerable work by Center investigators) has focused on an antidepressant-like influence of BDNF in hippocampus or frontal cortex. While we continue to analyze the importance of these findings (via other grants), we have provided novel evidence for a very different role played by BDNF and its signaling cascades acting at the level of the VTA-NAc circuit. This work has taken advantage of new tools, developed in conjunction with the Transgenic Core, which enable the selective knockout of BDNF signaling in the VTA, NAc, or other brain regions. This research has revealed that, in the VTA-NAc reward circuit, BDNF mediates a pro-depressant-like effect, with loss of BDNF selectively in this circuit mediating an antidepressant-like effect, in several animal models in both rats and mice. The goal of the proposed studies is to further establish the role played by BDNF in the VTA-NAc as it relates to mood, motivation, and depression, and to begin to identify the molecular substrates through which BDNF produces these novel effects. For example, recent findings from DNA expression arrays and behavioral studies have implicated several specific BDNF signaling proteins (in particular, the PI-3-kinase/Akt cascade) as key mediators of the BDNF behavioral phenotype in the VTA-NAc. We are now systemically analyzing the influence of these signaling proteins in animal models of depression and antidepressant action. We have also established BDNF and certain of its signaling proteins as key regulators of the morphology of VTA and NAc neurons, which we have demonstrated is altered by chronic exposure to stress, and we are interested in relating these findings to behavior. A relationship between BDNF and CREB is another focus of Project 2. We hypothesize that regulation of CREB activity is a major functional output of BDNF signaling in the VTA- NAc pathway. Conversely, we have growing evidence that CREB in turn regulates the VTA-NAc circuit in part via its control of BDNF gene expression. We are now using the unique molecular tools available to this Center to explore these possibilities as well as their detailed underlying molecular mechanisms.